This invention relates to a laminate composite sheet or strip of rubber compound which is made up of several separate layers of specifically designed rubber compound. The laminate may be made by any of the known methods, such as calendering and the like; however, it is preferred that the laminate be formed by a process known as coextrusion in which two or more rubber compounds passed through the preform die to form separate layers which are joined in the final die. A recent technique for this is disclosed in U.S. Pat. Nos. 3,479,425 and 3,557,265. This coextrusion process has been applied to plastics and thermal plastic elastomers to form laminates thereof.
Laminated articles are known in which the separate layers comprise materials which have different properties. Such articles have been utilized in the manufacture of tires wherein a stiff, partially cured rubber compound has been sandwiched between two layers of soft, tacky, uncured rubber compound by calendering the soft layers onto the already partially cured compound. Strips of this type have been employed in the bead area of the tire, where the tire contacts the rim, as an abrasion gum strip to resist the chafing that takes place between the tire and the rim.
An uncured rubber compound tends to flow during the curing operation, thereby descreasing its effective gauge or thickness. This is particularly true in the bead area and innerliner area of the tire where the curing pressure is high. In the above described type of composite strip the partial precure of the stiff compound enables it to retain its gauge or thickness during the tire curing operation, but the precure renders the strip inherently dry, causing an adhesion problem between it and the other parts of the tire during the building and curing operation. The external layers of the soft, tacky, uncured rubber compund in the laminate help to overcome this deficiency and provide the necessary uncured adhesion so that the composite strip will not separate from the remaining pieces of the tire prior to its final cure. However, there is still an adhesion problem because layers (the external layers) are being adhered to a partially cured layer.
Additionally, these prior composite strips have been difficult and expensive to manufacture. The process has been to calender the inner, stiff strip; subject it to a curing operation in which it is partially cured; and then calender the soft, tacky strips on both sides of the then stiff strip to form the final composite laminate. This operation has been expensive and time consuming in the past and involves several steps. Due to the limitations of the calendering operation, it has also necessitated the use of thicker strips for an adequate safety margin than are necessary to perform the functions in the final product.
The method and product of this invention has greatly simplified and improved the composite strip technology thereby enabling the use of composite strips not only in previous areas, but also in areas which were not feasible prior to this development. Strips employing this invention can be utilized in pneumatic tires as the air impervious liner which covers the inner periphery of the tire or as the abrasion gum strip which is located in the bead area of the tire that comes into contact with the flange of the tire rim when the tire is mounted and inflated.
The technology and invention of this application are not limited to these two places in a pneumatic tire. The technology may be applied to any of several end products, such as conveyor belts and industrial products.
The method and product of the invention are particularly useful where it is necessary to have one material retain a certain thickness in the end product. This has usually been accomplished in the past by using an excess amount of material to insure that the minimum is present in the final product after it has thinned out in the processing steps. The prior partial pre-cure method, although a help, still had this deficiency because only a partial cure could be obtained; a full cure would have rendered the cured stock unusable due to its poor adhesion.
This invention provides an improved laminate by selectively altering some of the layers in the laminate so that the layers will either be partially cured or fully cured when subjected to irradiation or will be uneffected by irradiation and remain uncured.
This technique eliminates at least one of the steps necessary in the prior processes. In this technique the composite strip is obtained by calendering or, preferably, by coextrusion. The rubber compounds in the various layers have been selectively either sensitized or desensitized to react to irradiation in various degrees. The composite strip is then subjected to irradiation wherein the sensitized layer or layers are partially or fully cured and the desensitized layer or layers are unaffected. Alternatively, all the layers may be sensitized to different degrees so that the composite has all of its layers cured to some degree with different layers cured to different degrees. This results in the different layers in the composite having different physical characteristics which may be utilized in the ultimate manufacture of the end product, as the abrasion gum strip application outlined above.
It is an object of this invention to provide a composite strip which can be manufactured in a relatively cheap and uncomplicated manner.
It is a further object of this invention to provide a composite strip in which the thickness of the separate layers is dictated by the amount of the layer necessary to perform its function and not by the problems inherent in the manufacture of the strips.
It is a further object of this invention to provide a composite strip in which some of the layers are sensitized to partially cure or totally cure when subjected to irradiation and other layers are desensitized to be unaffected by irradiation.
It is a further object of this invention to provide a composite strip in which all of the layers are sensitized to partially or totally cure but certain layers are sensitized to different degrees so that the physical characteristics of the layers differs.
These and other objects will be evident from the detailed description of the invention which follows.